Quantitative PET: Count Number Adaptations of Organ and Site specific Acquisition Protocols are a Key Determinant of comparable PET/CT Measurements

Aim: The ever-growing diversity and complexity of PET/CT systems make it increasingly difficult to define common acquisition protocols for quantifiable and comparable PET measurements in clinical routine and, above all, in multicenter clinical trials. We aimed to analyze the dependency of quantitative PET on technical variability, in order to improve acquisition and reconstruction protocols. The results formed the base for selected clinical acquisitions protocols, tailored to particular PET/CT systems. Materials and Methods: Image noise was examined in 422 PET/CT datasets from 18 PETCT systems participating in a Swiss multicenter phantom study. 215 of these measurements contained also hot spheres for recovery curve (RC) analysis. The study protocol combined different acquisition durations and low- and high-resolution image reconstructions with filtered back projection (FBP), ordered subset expectation maximization (OSEM), and vendor specific point spread function (PSF) based reconstruction. Data was analyzed with regard to exposure, defined as the product of background activity concentration and acquisition time. This produced results comparable in relation to the number of available decays per volume, independently from the actual activity concentration. Quality assurance (QA) limits for RCs were taken from guidelines issued by the Federal Office of Public Health in Switzerland. Additionally, RC shapes were quantitatively analyzed. Results: Passing the given QA limits depended highly on adequate exposure by keeping image noise levels low. Also, the minimal exposures required to fulfill the given QA limits differed between PET/CT systems, acquisition protocols and reconstruction algorithms. Despite higher image noise levels, quantifiability of FBP data was affected less by low count numbers than OSEM or PSF data. Insufficient count numbers led to faulty and incomparable image quantification,and consequently also erratic RCs. From the gathered data, we were able to propose limits for minimal exposure for ten different PET/CT devices and for common clinical protocols, suitable for whole body and organ-specific high-resolution acquisitions. Conclusions: In order to reproducibly generate comparable and quantitative data, count numbers in PET/CT acquisitions must be specifically adapted to the clinically necessary image resolution, reconstruction method and the PET/CT system used. Therefore, optimal PET/CT acquisition times vary with the required exposure and with the expected activity uptake in target organs. This in turn argues against fixed acquisition times and mandates site and organ specific acquisitions protocols. Adapting exposure to a site’s distinct technical factors appears to be a relevant element of PET/CT standardization in the context of multi-center trials